feat(network): add protocol packet capture and replay system

Add a recording and replay foundation for the MHF network protocol.
A RecordingConn decorator wraps network.Conn to transparently capture
all decrypted packets to binary .mhfr files, with zero handler changes
and zero overhead when disabled.

- network/pcap: binary capture format (writer, reader, filters)
- RecordingConn: thread-safe Conn decorator with direction tracking
- CaptureOptions in config (disabled by default)
- Capture wired into all three server types (sign, entrance, channel)
- cmd/replay: CLI tool with dump, json, stats, and compare modes
- 19 new tests, all passing with -race

server/signserver/session.go

@@ -27,12 +27,13 @@ const (
 // Session holds state for the sign server connection.
 type Session struct {
 	sync.Mutex
-	logger    *zap.Logger
-	server    *Server
-	rawConn   net.Conn
-	cryptConn *network.CryptConn
-	client    client
-	psn       string
+	logger         *zap.Logger
+	server         *Server
+	rawConn        net.Conn
+	cryptConn      network.Conn
+	client         client
+	psn            string
+	captureCleanup func()
 }
 
 func (s *Session) work() {